With the increasingly widespread use of fiberoptic cables as wideband data links, it has become increasingly important to have instruments which are able to accurately locate and measure the optical characteristics and parameters of such cables. Among these characteristics and parameters are characteristics such as attenuation, cutoff, and polarization, and parameters such as the length of the fiber, the location of the end of the fiber, and the numbers, magnitudes and locations of lossy features such as couplings, splices, and defects, among others. Instruments which are designed to perform such measurements are referred to as Optical Time Domain Reflectometers or, more commonly, OTDRs. An example of one type of OTDR is described in U.S. Pat. No. 4,794,249 (Beckmann et al). Examples of commercially available ones of such OTDRs are sold under the model designations TD-1000 and TD-3486 OTDRs by the Laser Precision Division of GN Nettest of Utica, N.Y.
The precision and accuracy of OTDR measurements can be affected by various noise sources. Some of these are purely electronic in nature and others are directly related to the production, transmission, reflection and detection of the optical signal. Among the most important types of noise that affect transmission through fiberoptic cables are background or "white" noise and coherence/polarization noise. Background noise is essentially random in character. This type of noise exists in all systems that include electronic circuitry and affects all pulses, without regard to the temporal duration or spectral width thereof. Because optical signals transmitted along fiberoptic cables decrease in amplitude with distance, they tend to become difficult to distinguish from this noise. OTDRs cope with this type of noise by basing final results on the average of the results of many individual measurements. This is because averaging is a process that tends to cancel out the randomly varying components of a signal such as background noise.
Coherence/polarization noise, on the other hand, is not random in character. It is caused by microscopic features in the optic fiber, such as impurities and variations in dopant concentration. Since such features are localized at fixed points along the fiber, they tend to have repeatable effects. The magnitude of this noise is not particularly troublesome for relatively long duration pulses, such as those with a temporal width significantly greater than the coherence time of the source. This is because such noise tends to become "washed out" over the course of a pulse. For relatively short duration pulses, such as those with a temporal width less than the coherence time, on the other hand, the effect is unable to "wash out" over the course of a pulse. Moreover, because the effect is coherent rather than random, it is less subject to being removed by averaging. Prior to the present invention, attempts to reduce the magnitude of coherence/polarization effects have made use of one of two approaches. One of these involves the use of hardware and software implemented filtering applied on a post sampling basis, i.e., on filters applied to signals after they have been both transmitted and received. A second of these approaches involves using lasers with larger than usual cavities, although this approach is effective in some but not all cases.
For the sake of brevity, the phrase "coherence/polarization noise" will hereinafter be abbreviated to "coherence noise".
Attempts to deal with coherence noise by means of the post processing of the received signal have not been entirely successful. This is because the filtering process used in this post processing often has the effect of filtering out weak but significant events. As a result, the filtering process can prevent the detection of the very features that the measurement is performed to detect and, what is worse, prevent such detection on an intermittent and unpredictable basis. It has recently become evident that the degree of this unpredictability can vary from laser to laser within or between manufacturing batches thereof, as well as from fiber to fiber.
In view of the foregoing it will be seen that, prior to the present invention, there has existed a need for a method and apparatus for reducing the effect of coherence noise in OTDRs, and for doing so in a manner that does not effect the stability and repeatability of measurements made with ODTRs.